doc: tweak Go 1.4 release notes
doc: tweak Go 1.4 release notes

LGTM=dave, dsymonds
R=golang-codereviews, dave, dsymonds
CC=golang-codereviews
https://golang.org/cl/184350043

zhangg/go-1,zhangg/go-1,zhangg/go-1,zhangg/go-1,zhangg/go-1,zhangg/go-1,zhangg/go-1,zhangg/go-1